The present invention relates to a liquid crystal display and a method for fabricating the same, more specifically a liquid crystal display of homeotropic alignment mode in which directions of inclination of liquid crystal molecules upon application of a voltage is controlled by the use of projections in the shape of banks provided on the surfaces of the substrates.
As an LCD (Liquid Crystal Display) using an active matrix, a liquid crystal display of TN (Twisted Nematic) mode in which a liquid crystal material having positive dielectric constant anisotropy is aligned horizontal with respect to the substrate plane and twisted at 90xc2x0 between the opposed substrates is conventionally widely used. However, the liquid crystal display of the TN mode has a serious disadvantage of poor visual angle characteristics. To improve the visual characteristics, various studies have been made.
In view of this, the inventors of the present application have made earnest studies and proposed a liquid crystal display of MVA (Multi-domain Vertical Alignment) mode which will take over the TN mode, in which a liquid crystal material having a negative dielectric constant anisotropy is homeotropically aligned, and directions of inclination direction of the liquid crystal molecules upon application of a voltage are restricted by structures, e.g., bank-shaped projections provided on the surfaces of the substrates, whereby they have succeeded in obtaining drastically improved visual angle characteristics (refer to, e.g., the specification of Japanese Patent Application No. 361384/1997 filed by the applicant of the present application).
As shown in FIG. 42A, the liquid crystal display of the above-described MVA mode in which a liquid crystal material having negative dielectric constant anisotropy is vertically aligned includes bank-shaped projections of a light transmitting resin (e.g., resist or others) provided on the substrates, so that the liquid crystal molecules are restricted to be aligned, inclined in a plurality of directions in one picture element when a voltage is applied, whereby improved visual angle characteristics can be obtained.
However, the liquid crystal display of the above-described conventional MVA mode, which has the projections formed in the display picture element region, cannot avoid, in principle, aperture ratio decrease. The transmittance in the bright state is lower in comparison with the TN mode.
That is, in the MVA mode, a voltage applied to the liquid crystal molecules in the projection formed regions is lowered. Accordingly, as shown in FIG. 42B, the liquid crystal molecules at the projection summits are not inclined, and resultantly all the liquid crystal molecules on the projections are not easily inclined. When the liquid crystal molecules on the projections are inclined upon application of a voltage, a direction of the inclination is normal to the projections, i.e., substantially identical with an inclination direction of the liquid crystal molecules in the gaps. Accordingly, a transmittance curve of the light passing through the liquid crystal panel is as shown in FIG. 42C, and an aperture ratio is decreased by an area of the projections.
Various mechanisms for achieving the alignment control are known. Generally it is accepted that an equipotential surface of the liquid crystal layer is deformed by the bank-shaped projections upon application of a voltage, and the liquid crystal is aligned slantly backward at both edges of the bank-shaped projections (refer to FIG. 43A). On the other hand, when alignment restricting forces are equal to each other on both edges, the liquid crystal on the bank-shaped projections is tilted stable in a direction displaced from the edges by a 90xc2x0 or a 180xc2x0 azimuth, i.e., in a direction parallel with extension of the bank-shaped projections.
As a film thickness of a light transmitting resin, i.e., a height of the bank-shaped projections is increased, a transmittance in the white state is increased, but leakage light is generated in the black state. Accordingly, the contrast is poor. The liquid crystal display of the MVA mode was originally developed for monitor purposes, and priority was given to contrast in setting a height of the bank-shaped projections.
The liquid crystal display of the MVA mode, in which the alignment control structures in the form of the bank-shaped projections are formed on the picture element, and the liquid crystal is aligned, inclined to a prescribed direction at the edges of the projections, has lower transmittance in the white state in comparison with the TN panel. That is, the above-described conventional liquid crystal display gives priority to contrast in setting a height of the bank-shaped projections, and does not have the transmittance optimized with a result of low transmittance due to generation of alignment disorder. Specifically, the liquid crystal alignment on the bank-shaped projections of the liquid crystal display of the MVA mode was observed, and the liquid crystal tilted in reverse directions (90xc2x0 and 180xc2x0) to form constricted domains (inverse tilt domains). Alignment disorders were generated from the constricted domains to the outsides of the banks (refer to FIG. 43B). Such liquid crystal alignment disorders have resulted in the low transmittance.
In considering of electric power saving of the panel and incorporation of the panel in notebook-type personal computers, improvement of the brightness of the liquid crystal display of the MVA mode is a significant problem. It is expected that transmittance decrease on the bank-shaped projections and at the edges thereof will be minimized, and the liquid crystal molecules in the gaps will be easily tilted when a voltage is applied.
An object of the present invention is to provide a liquid crystal display which improves luminance of a liquid crystal display of the MVA-mode and a method for fabricating the same.
The above-described object is achieved by a liquid crystal display including: a first substrate including an active device for driving a liquid crystal, a picture element electrode to which a drive voltage is applied by the active device, and a first alignment film formed on the picture element electrode and aligning liquid crystal molecules vertically to a surface of the first alignment film when the drive voltage is not applied; a second substrate including a common electrode opposed to the picture element electrode, and a second alignment film formed on the common electrode and aligning the liquid crystal molecules vertically to a surface of the second alignment film when the drive voltage is not applied; and a liquid crystal layer sealed between the first substrate and the second substrate and having negative dielectric constant anisotropy, the liquid crystal display comprising: a bank-shaped projection for restricting a tilting direction of the liquid crystal molecules when the drive voltage is applied, disposed between the picture element electrode and the first alignment film, and/or between the common electrode and the second alignment film, the first alignment film and/or the second alignment film having weaker vertical alignment restrictiveness in a region where the bank-shaped projection is formed than that in a region where the bank-shaped projection is not formed.